diff --git a/pyproject.toml b/pyproject.toml
index 2db9b32a..54c9231d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,6 +58,9 @@ processing = [
     "fasteners",
     "xxhash"
 ]
+decont = [
+    "lighteval>=0.3.0"
+]
 quality = [
   "ruff>=0.1.5"
 ]
@@ -66,6 +69,7 @@ testing = [
   "datatrove[io]",
   "datatrove[processing]",
   "datatrove[s3]",
+  "datatrove[decont]",
   "pytest",
   "pytest-timeout",
   "pytest-xdist",
diff --git a/src/datatrove/io.py b/src/datatrove/io.py
index be99291e..f91f2c77 100644
--- a/src/datatrove/io.py
+++ b/src/datatrove/io.py
@@ -284,6 +284,11 @@ def open_file(file: IO | str, mode="rt", **kwargs):
     return file
 
 
+def file_exists(path: str):
+    fs, a, fpath = get_fs_token_paths(path)
+    return fs.exists(fpath[0])
+
+
 def download_file(remote_path: str, local_path: str, progress: bool = True):
     fs, _, paths = get_fs_token_paths(remote_path)
     fs.get_file(
diff --git a/src/datatrove/pipeline/decont/__init__.py b/src/datatrove/pipeline/decont/__init__.py
new file mode 100644
index 00000000..efea634c
--- /dev/null
+++ b/src/datatrove/pipeline/decont/__init__.py
@@ -0,0 +1 @@
+from .n_grams import NGramsDecontConfig, NGramsDecontFilter, NGramsDecontIndexer
diff --git a/src/datatrove/pipeline/decont/n_grams.py b/src/datatrove/pipeline/decont/n_grams.py
new file mode 100644
index 00000000..64b25d22
--- /dev/null
+++ b/src/datatrove/pipeline/decont/n_grams.py
@@ -0,0 +1,220 @@
+"""
+Used for n-gram decontamination.
+First build an index using the tasks we want to use to decontaminate our training dataset.
+Then read your training data and apply the filter with the index loaded.
+"""
+
+import os
+from collections import defaultdict
+from concurrent.futures import ThreadPoolExecutor
+from dataclasses import dataclass, field
+from typing import Tuple
+
+import numpy as np
+from loguru import logger
+
+from datatrove.data import Document, DocumentsPipeline
+from datatrove.io import DataFolderLike, file_exists, get_datafolder, open_file
+from datatrove.pipeline.base import PipelineStep
+from datatrove.pipeline.filters.base_filter import BaseFilter
+from datatrove.pipeline.writers.disk_base import DiskWriter
+from datatrove.utils.binaryio import read_np_from_file
+from datatrove.utils.text import TextNormConfig, simplify_text, xxhash64
+
+
+@dataclass
+class NGramsDecontConfig:
+    """
+    Example for n_grams=4
+    query = ['A', 'B', 'C', 'D', 'E'] (the prompt/instruction)
+    label = ['F', 'G', 'H', 'I', 'J'] (the answer/gold)
+    Will find the following N-GRAMS in the training data:
+        'F G H I'
+        'G H I J'
+        + IF find_query_ngrams:
+            'A B C D'
+            'B C D E'
+        + IF find_overlap_ngrams:
+            'C D E F'
+            'D E F G'
+            'E F G H'
+    """
+
+    n_grams: int = 12
+    find_query_ngrams: bool = False  # enable to also check for matches in n-grams containing only the input/prompt
+    find_overlap_ngrams: bool = True  # will also find matches for n-grams containing BOTH input and query
+    norm_config: TextNormConfig = field(default_factory=TextNormConfig)
+
+
+DEFAULT_NGRAMS_DECONT_CONFIG = NGramsDecontConfig()
+
+
+class NGramsDecontIndexer(PipelineStep):
+    """
+    Creates a decontamination index (basically a list of uint64 hashes from ngrams) for each reference task.
+    Ways to provide task data:
+      - as input documents from the previous pipeline step with "text=label/correct answer"
+        and metadata={"query": query/prompt/input, "task": task name}
+      - as a list of strings in the format "suite|task" from the lighteval metadata table:
+      https://github.com/huggingface/lighteval/blob/main/src/lighteval/tasks/tasks_table.jsonl as `lighteval_tasks`
+      - a path to a text file containing one such list, with one "suite|task" per line as `lighteval_tasks`
+      you can also define your custom tasks with `custom_lighteval_tasks`. See explanation for `custom_tasks` here:
+      https://github.com/huggingface/lighteval/tree/main?tab=readme-ov-file#evaluate-a-model-on-extended-community-or-custom-tasks
+
+    """
+
+    type = "🦠 - DECONT"
+    name = "💥 N-grams build index"
+    _requires_dependencies = ["nltk", "lighteval", "xxhash"]
+
+    def __init__(
+        self,
+        output_folder: DataFolderLike,
+        lighteval_tasks: str | list[str] | None = None,  # list in the format suite|task or path to one such list
+        custom_lighteval_tasks: str | None = None,
+        config: NGramsDecontConfig = DEFAULT_NGRAMS_DECONT_CONFIG,
+        language: str = "english",
+    ):
+        super().__init__()
+        self.output_folder = get_datafolder(output_folder)
+        # parse list of tasks
+        if isinstance(lighteval_tasks, str):
+            if file_exists(lighteval_tasks):
+                with open_file(lighteval_tasks, "rt") as f:
+                    self.lighteval_tasks = f.read().strip().splitlines()
+            else:
+                self.lighteval_tasks = [lighteval_tasks]
+        else:
+            self.lighteval_tasks = lighteval_tasks
+        self.custom_lighteval_tasks = custom_lighteval_tasks
+        self.config = config
+        self.language = language
+
+    def compute_hashes(self, label: str, query: str | None = None) -> list[int]:
+        from nltk import ngrams
+        from nltk.tokenize import word_tokenize
+
+        label_tokens = word_tokenize(simplify_text(label, self.config.norm_config), language=self.language)
+        ngrams_to_compute = list(ngrams(label_tokens, self.config.n_grams))
+        if query is not None:
+            query_tokens = word_tokenize(simplify_text(query, self.config.norm_config), language=self.language)
+            if self.config.find_query_ngrams:
+                ngrams_to_compute.extend(ngrams(query_tokens, self.config.n_grams))
+            if self.config.find_overlap_ngrams:
+                # add tokens overlapping query and label
+                """
+                A, B, C, D, E | F, G, H, I, J
+                5 grams
+                B, C, D, E, F (-N + 1 + i:) + (:i + 1)
+                ...
+                E, F, G, H, I
+                """
+                ngrams_to_compute.extend(
+                    [
+                        query_tokens[-self.config.n_grams + 1 + i :] + label_tokens[: i + 1]
+                        for i in range(self.config.n_grams - 1)
+                        # make sure we actually get a list of size N
+                        if len(query_tokens) >= self.config.n_grams - 1 - i and len(label_tokens) >= i + 1
+                    ]
+                )
+        return list(map(xxhash64, map(" ".join, ngrams_to_compute)))
+
+    def run(self, data: DocumentsPipeline = None, rank: int = 0, world_size: int = 1):
+        if world_size != 1:
+            raise ValueError("Decontamination index building requires a single worker.")
+        hashes = defaultdict(set)
+        # use whatever date is parsed in with the following format:
+        # doc.text -> label
+        # doc.metadata["input"] -> input
+        if data:
+            for doc in data:
+                if not self.config.find_query_ngrams and "query" not in doc.metadata:
+                    raise ValueError(
+                        "only_label_ngrams is False but could not find 'query' field in documents metadata"
+                    )
+                hashes[doc.metadata.get("task", "input")].update(
+                    self.compute_hashes(doc.text, doc.metadata.get("query", None))
+                )
+
+        # parse data from lighteval defined tasks
+        from lighteval.tasks.lighteval_task import LightevalTask
+        from lighteval.tasks.registry import Registry
+
+        task_dict = Registry(cache_dir=os.getenv("HF_HOME")).get_task_dict(
+            self.lighteval_tasks, custom_tasks=self.custom_lighteval_tasks
+        )
+        LightevalTask.load_datasets(task_dict.values())
+
+        for task_name, task in task_dict.items():
+            for eval_doc in task.eval_docs():
+                for gold in eval_doc.get_golds():
+                    hashes[task_name].update(self.compute_hashes(gold, eval_doc.query))
+
+        for task_name, task_hashes in hashes.items():
+            hashes_array = np.array(list(task_hashes), dtype="<u8")
+            logger.info(f"Saving {len(task_hashes)} hashes for {task_name}")
+            with self.output_folder.open(f"{task_name.replace(' ', '_')}.index.hashes", mode="wb") as f:
+                if self.output_folder.is_local():
+                    hashes_array.tofile(f)
+                else:
+                    f.write(hashes_array.tobytes())
+
+
+class NGramsDecontFilter(BaseFilter):
+    """
+    Loads list of hashes created by the Indexer step.
+    For each document in the block's input, we will check if any of its ngrams are part of the reference eval tasks.
+    If so, they will be removed. The contaminated ngram and task where it was found will be saved in the removed
+    document's metadata.
+    """
+
+    type = "🦠 - DECONT"
+    name = "💥 N-grams decontaminate"
+    _requires_dependencies = ["nltk", "xxhash"]
+
+    def __init__(
+        self,
+        index_folder: DataFolderLike,
+        config: NGramsDecontConfig = DEFAULT_NGRAMS_DECONT_CONFIG,
+        exclusion_writer: DiskWriter = None,
+        language: str = "english",
+    ):
+        super().__init__()
+        self.index_folder = get_datafolder(index_folder)
+        self.config = config
+        self.exclusion_writer = exclusion_writer
+        self.language = language
+        self._index_hashes = None
+
+    def load_index_hashes(self):
+        def load_index_from_file(file):
+            with self.index_folder.open(file, mode="rb") as f:
+                return file, read_np_from_file(f, np.dtype("<u8"), self.index_folder.is_local()).tolist()
+
+        with ThreadPoolExecutor() as pool:
+            hashes = pool.map(load_index_from_file, self.index_folder.list_files())
+
+        self._index_hashes = {}
+        for filename, hashlist in hashes:
+            taskname = filename.removesuffix(".index.hashes")
+            logger.info(f"Loading {len(hashlist)} hashes for {taskname}")
+            for hash in hashlist:
+                self._index_hashes[hash] = taskname
+
+    def filter(self, doc: Document) -> bool | Tuple[bool, str]:
+        if self._index_hashes is None:
+            self.load_index_hashes()
+
+        from nltk import ngrams
+        from nltk.tokenize import word_tokenize
+
+        text_tokens = word_tokenize(simplify_text(doc.text, self.config.norm_config), language=self.language)
+        ngrams_to_compute = list(ngrams(text_tokens, self.config.n_grams))
+        for n_gram in map(" ".join, ngrams_to_compute):
+            task = self._index_hashes.get(xxhash64(n_gram), None)
+            if task is not None:
+                doc.metadata["contaminated_ngram"] = n_gram
+                doc.metadata["contaminated_task"] = task
+                self.stat_update(f"contaminated_{task}")
+                return False, "contaminated"
+        return True
diff --git a/tests/pipeline/test_ngrams_decont.py b/tests/pipeline/test_ngrams_decont.py
new file mode 100644
index 00000000..e60e9305
--- /dev/null
+++ b/tests/pipeline/test_ngrams_decont.py
@@ -0,0 +1,60 @@
+import copy
+import shutil
+import tempfile
+import unittest
+
+from datatrove.data import Document
+from datatrove.pipeline.decont import NGramsDecontConfig, NGramsDecontFilter, NGramsDecontIndexer
+from tests.utils import require_xxhash
+
+
+TEXTS = [
+    "A lady walks to a barbell. She bends down and grabs the pole.",  # 0: contaminated query
+    "get into formation, then begin dancing and flipping as male cheerleaders join them.",  # 1: contaminated label
+    "He is using commercial lawn mowing equipment. he walks back and forth as he mows the grass.",  # 2: cont overlap
+    "He is using commercial lawn mowing equipment. he is animated as he does the task.",  # 3: incorrect completion
+    "walks outside plugs his lawn mower in and gets ready to mow",  # 4: single contaminated query ngram
+    "",  # 5: not contaminated at all
+    "walks outside plugs his lawn mower in and gets ready to",  # 6: single contaminated query text < 1 ngram
+]
+
+DOCS = [
+    Document(
+        text="Nothing is so painful to the human mind as a great and sudden change. "
+        + text
+        + " Beware; for I am fearless, and therefore powerful.",
+        id=str(text_i),
+    )
+    for text_i, text in enumerate(TEXTS)
+]
+
+
+@require_xxhash
+class TestNGramDecont(unittest.TestCase):
+    def setUp(self):
+        # Create a temporary directory
+        self.tmp_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, self.tmp_dir)
+
+    def get_test_results(self, config):
+        indexer = NGramsDecontIndexer(self.tmp_dir, lighteval_tasks="leaderboard|hellaswag", config=config)
+        indexer.run()
+        nfilter = NGramsDecontFilter(self.tmp_dir, config=config)
+        return tuple([int(doc.id) for doc in nfilter(copy.deepcopy(DOCS))])
+
+    def test_label_only(self):
+        self.assertEqual(
+            self.get_test_results(NGramsDecontConfig(find_query_ngrams=False, find_overlap_ngrams=False)),
+            (0, 2, 3, 4, 5, 6),
+        )
+
+    def test_query(self):
+        self.assertEqual(
+            self.get_test_results(NGramsDecontConfig(find_query_ngrams=True, find_overlap_ngrams=False)), (2, 3, 5, 6)
+        )
+
+    def test_overlap(self):
+        self.assertEqual(
+            self.get_test_results(NGramsDecontConfig(find_query_ngrams=False, find_overlap_ngrams=True)),
+            (0, 3, 4, 5, 6),
+        )
diff --git a/tests/utils.py b/tests/utils.py
index 65af1b3d..0fe4b7cd 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -103,3 +103,11 @@ def require_xxhash(test_case):
     except ImportError:
         test_case = unittest.skip("test requires xxhash")(test_case)
     return test_case
+
+
+def require_lighteval(test_case):
+    try:
+        import lighteval  # noqa: F401
+    except ImportError:
+        test_case = unittest.skip("test requires lighteval")(test_case)
+    return test_case